This invention relates to electromagnetic coils for electron lenses, particularly for use in electron microscopes, electron beam inspection systems and the like. Such coils are difficult to cool, which constrains the current density they can handle, and they are incompatible with the high vacuum environment in which they operate, due to the plastic insulator used on the coil wire and air trapped in the coil during the winding process. These coil limitations have in turn fettered the performance and raised the cost of the embodying magnetic lenses and the systems which incorporate such lenses.
To assure against overheating and consequent damage to the coil wire insulation, the current density in such coils must be kept low enough to maintain acceptable heat levels at the core of the coil. (Since wire insulation is a poor thermal conductor, heat generated in the interior of the coil sees a high thermal impedance path to the coil exterior.) But low current density necessitates undesirably large (and expensive) coil structures to achieve a desired lens power.
Secondly, because of the incompatibility of the coil with a high vacuum environment, due to outgassing of insulator hydrocarbons and escape of trapped air, such coils must be sealed from the high vacuum environment in which they operate.
One approach is to seal the coil from the high vacuum environment but couple it to air. An alternate approach is to hermetically encapsulate the lens coil within the vacuum environment and in that may isolate it from the high vacuum environment. In both approaches the lens coil is actually hermetically sealed from the electron beam passing through it and upon which it must act. Both approaches are undesirable, yielding somewhat inefficient, expensive and bulky arrangements.
Prior to this invention there did not exist an electromagnetic coil suitable for magnetic lenses, operable in a high vacuum environment at high efficiency and without causing significant degradation of the quality of its environment.